The invention is concerned with the issue of how alternative raw materials may be exploited in the production of C4-based aldehydes.
Hydrocarbons are chemical compounds which consist exclusively of carbon and hydrogen. Alkenes (synonym: olefins) are hydrocarbons which have a C═C double bond in the molecule. Alkanes (synonym: paraffins), on the other hand, are hydrocarbons which have only single bonds. They are therefore also referred to as saturated. Due to the different bond types, alkenes are significantly more reactive than alkanes. Therefore, alkenes are chemically more utilizable and correspondingly more valuable than alkanes.
In organic chemistry, hydrocarbons are frequently designated according to the number of carbon atoms which they have per molecule, in that the respective class of substances is preceded by the prefix Cn. “n” is the respective number of carbon atoms in a molecule. Thus, C4 olefins are substances from the class of alkenes having four carbon atoms. C8 olefins correspondingly have eight carbon atoms per molecule. Where the prefix Cn+ is used hereinafter, it refers to a class of substances which have more than n carbon atoms per molecule. A C4+ olefin accordingly has at least five carbon atoms.
Due to the different arrangement and linking possibilities of the carbon and hydrogen atoms, several isomers, which have the same number of carbon atoms, exist within the substance classes discussed here. For instance, two alkanes exist having four carbon atoms in each case, namely n-butane and isobutane. Since the variety of combinations is greater for the alkenes, even more isomers are possible. For instance, in total four olefins having four carbon atoms exist, namely isobutene, 1-butene, cis-2-butene and trans-2-butene. The three linear butenes, 1-butene, cis-2-butene and trans-2-butene, are often referred to collectively as n-butene. For the C3 hydrocarbons in contrast, there is only one isomer in each case, namely the alkane having three carbon atoms, propane, and the C3 alkene propene. In the longer-chain C5+ hydrocarbons, the multiplicity of isomers increases markedly. Despite the identical number of carbon atoms, isomers have different properties which are relevant for their industrial use.
The aldehyde substance class comprises substances which, due to their high reactivity, are used as starting substance for the preparation of various speciality chemicals such as lubricants, plasticizers and detergents. It is also possible to use aldehydes as fragrances.
Aldehydes are produced from alkenes and synthesis gas, i.e. a mixture of hydrogen and carbon monoxide. This procedure is called hydroformylation or oxo reaction. In this case, the number of carbon atoms increases by one. In this manner, a C5 aldehyde (pentanal) is formed from a C4 olefin by hydroformylation.
Aldehydes having a higher carbon atom number can be generated by either reacting lower aldehydes with one another to give higher aldehydes (aldol condensation) or reacting olefins only with themselves (oligomerization) and then hydroformylating the olefin oligomers obtained in this case.
For instance, a C10 aldehyde may be obtained by hydroformylating a C4 olefin to give the C5 aldehyde and this is then reacted with itself by aldol condensation to give the C10 aldehyde (decanal). C9 aldehyde may also be prepared from C4 olefin if it is firstly converted by oligomerization to a C8 olefin and this is subsequently hydroformylated to the C9 aldehyde.
Aldehydes having both five and nine carbon atoms can thus be prepared from C4 olefins; also C10 aldehydes by subsequent aldol condensation of the pentanals. This is also thus carried out in industrial practice in complexly connected compound installations:
DE102008007081A1 describes a process for utilizing C4 mixtures comprising at least 1-butene, isobutene, butanes, 2-butenes and polyunsaturated C4 hydrocarbons. It is mentioned in passing that C9, C13 and C17 aldehydes can be prepared from the separated n-butene via oligomerization and hydroformylation, while the high-purity 1-butene also obtained is suitable, inter alia, for preparing valeraldehyde.
This process uses as raw material source so-called C4 cuts which originate as “crack C4” from steamcrackers or as “FCC C4” from fluidized-catalytic crackers. Such crackers are substantially charged with naphtha or VGO (vacuum gas oil) which originate in turn from the distillation of crude oil. Since crack C4 and FCC C4 are in the added-value chain of the petrochemical products of crack processes, the prices for these raw materials are correspondingly volatile owing to their dependence on the price of oil. Moreover, the availability of high-value crack C4 has been steadily falling since the operation of the steam crackers is optimized towards the production of the C2 and C3 olefins ethene and propene to the detriment of the C4 yield. A disadvantage of the process described in DE102008007081A1 can thus be considered to be its dependence on a specific raw material basis.
A further disadvantage of this process is that the n-butane and isobutane, sometimes present in significant amounts in C4 mixtures used, exhibit inert behaviour in the process and are therefore not materially utilized. In the interests of the CO2 balance of the process, as far as possible all carbon atoms present in the feedstock mixture should be utilized in a chemically sustainable manner and if at all possible they should not be incinerated. The resource efficiency of the process known from DE102008007081A1 is therefore capable of improvement.
Another raw material basis uses the process described in EP0820974B1. Processed therein are so-called “field butanes” which are C4 fractions of the “wet” components of natural gas and the gases accompanying mineral oil, which are separated from the gases by drying and cooling to about −30° C. in liquid form. Low-temperature distillation gives the field butanes whose composition fluctuates depending on the deposit, but which generally comprise about 30% isobutane and about 65% n-butane. Further constituents are generally about 2% hydrocarbons having fewer than four carbon atoms and about 3% C4+ hydrocarbons.
The field butanes are dehydrogenated such that a mixture is formed comprising, inter alia, n-butene and isobutene. This mixture is worked-up and the n-butene separated here is converted by oligomerization to substantially C8 olefins and additionally C12 olefins. The C8 and C12 olefins are converted by hydroformylation and hydrogenation into C9 and C13 alcohols, and therefore corresponding aldehydes must be present prior to hydrogenation. C5 aldehydes are not produced however. A disadvantage of this process is that a continuous turnover of C9 and C13 alcohols is required in order to be able to utilize the field butanes in question. Since field butanes can hardly be traded otherwise, the purchase of field butanes is possible only via long-term continuous supply contracts. Therefore, there is a dependence on a specific raw material here also which is enhanced by buyer dependence, however.
With respect to EP0820974B1, there is therefore the need, through technical measures, to have a greater freedom in the choice of raw material suppliers and to be able to react to fluctuating demand of the buyers.
Another raw material source for aldehyde preparation is in turn exploited in US2006/0122436A1. The alkanes used in this publication originate from LPG.
LPG (liquefied petroleum gas) is a common international trade name for a liquid mixture of C3 and/or C4 hydrocarbons which is obtained as a by-product in the recovery of mineral oil or natural gas from its deposit or in the work-up of crude oil in the refinery. The precise composition of LPG depends significantly on its origin; its essential constituents are usually propane and butanes.
There exists a global ecosystem based on LPG which promotes, transports and markets this product mainly as propellant and fuel.
A comprehensive introduction to LPG technology and economics based thereon is found in:                Thompson, S. M., Robertson, G. and Johnson, E.: Liquefied Petroleum Gas. Ullmann's Encyclopedia of Industrial Chemistry. Published Online: 15 Jul. 2011. DOI: 10.1002/14356007.a15_347.pub2        
US2006/0122436A1 discloses, then, two routes as to how aldehydes or alcohols can be prepared from LPG; cf. the claims 1 and 7 therein.
In the first process according to claim 1, Cn aldehydes are produced from Cn-1 alkanes. n in this case is an integer from 4 to 20. Accordingly, in the case of n=5, C5 aldehydes are prepared from C4 alkanes, and, in the case of n=9, C9 aldehydes are prepared from C8 alkanes. This is accomplished, for example in the case of C4 alkanes, by dehydrogenating the butane present in the LPG initially to butene and secondary constituents. After removal of the secondary constituents, the butenes are hydroformylated to pentanals. The pentanals are converted by aldol condensation into decanals.
In the second process, the corresponding C2n and C2n-1 alcohols are prepared from Cn-1 alkanes via Cn aldehydes, C2n aldehydes and C2n-1 aldehydes. In the case of n=5, C5, C9 and C10 alcohols are accordingly produced from C4 alkanes. This is accomplished in principle exactly as in the first process but with the difference that the hydroformylation, for example of the C4 alkenes, to give the pentanals takes place explicity only under partial conversion; cf. step 7c. The pentanals are separated from the unreacted butenes (step 7d) and aldol-condensed to give decanals, in order to subsequently prepare decanols by means of catalytic hydrogenation (steps 7e and 7f). The unreacted butenes are subjected to an oligomerization (step 7g) such that C8 olefins are obtained. These are then hydroformylated to give C9 aldehydes (step 7h) and subsequently hydrogenated to give C9 alcohols (step 7i).
A conceptual disadvantage of this process is that the oligomerization (from Cn-1 to C2n-2) and the subsequent second hydroformylation of the oligomers (from C2n-2 to C2n-1) is arranged downstream of the first hydroformylation (from Cn-1 to Cn), the two hydroformylation steps thus being connected serially (in series). This means that the second hydroformylation is provided as a “residue utilization” of the first hydroformylation and ultimately converts the oligomers of the Cn-1 alkenes which the first hydroformylation did not convert. The supply to the second hydroformylation with raw material is accomplished consequently by adjusting the degree of the partial conversion of the first hydroformylation.
In a market situation in which significantly more C9 aldehydes than C5 or C10 aldehydes are in demand, the conversion of the first hydroformylation (which serves the C5 and C10 market) has to be very significantly shut down in the compound concept described in US2006/0122436A1, in order to leave sufficient unreacted butene for the second hydroformylation (which makes the desired C9). This means that the first hydroformylation has to be conducted in a very unfavorable operating state and therefore operates very inefficiently.
A further disadvantage of the serial connection of dehydrogenation, first hydroformylation, oligomerization and second hydroformylation is due to the fact that commercially available plants for the dehydrogenation of alkanes, which is mentioned in paragraph [0022] of US2006/0122436A1, are operated generally in the context of naphtha crackers such that these processes are all designed and optimized on a throughput in the petrochemical dimension. For instance, the capacity of a propane dehydrogenation according to the STAR® process is about 500 000 t/a of propylene. Propane dehydrogenations by the CATOFIN® process are even designed for 850 000 t/a. These are scales which differ very markedly from those of industrially operated hydroformylation; thus the capacity of an oxo plant is typically only 100 000 t/a. Even if two large 250 kt/a oxo plants were capable of processing the alkenes supplied from a 500 kt/a dehydrogenation, the first hydroformylation would still have to be additionally oversized in order to be able to loop a correspondingly large amount of unreacted alkene for the second oxo plant in the case of partial loading. Using this process layout, the dehydrogenation is therefore too large or the hydroformylations are too small in order to be able to operate the entire process economically. A remedy here would offer only a costly specific development of an unusually small dehydrogenation or the possibility to use alkene produced in excess in some other way than for aldehyde production. In this respect, once again new buyer dependencies are created.